Expression of the Azotobacter vinelandii poly-β-hydroxybutyrate biosynthetic phbBAC operon is driven by two overlapping promoters and is dependent on the transcriptional activator PhbR

The Azotobacter vinelandii phbBAC genes encode the enzymes for poly-beta-hydroxybutyrate (PHB) synthesis. The phbR gene, which is located upstream of and in the opposite direction of phbBAC, encodes PhbR, a transcriptional activator which is a member of the AraC family of activators. Here we report that a mutation in phbR reduced PHB accumulation and transcription of a phbB-lacZ fusion. We also report that phbB is transcribed from two overlapping promoters, p(B)1 and p(B)2. The region corresponding to the -35 region of P(B)1 overlaps the P(B)2-10 region. In the phbR mutant, expression of phbB from the P(B)1 promoter is significantly reduced, whereas expression from the P(B)2 promoter is slightly increased. Two phbR promoters, P(R)1 and P(R)2, were also identified. Transcription from PR2 was shown to be dependent on sigma(S). Six conserved 18-bp sites, designated R1 to R6, are present within the phbR-phbB intergenic region and are proposed to be putative binding targets for PhbR. RI overlaps the -35 region of the p(B)1 promoter. A model for the regulation of phbB transcription by PhbR is proposed.